Owing to their excellent properties such as transparency, heat resistance, gas-barrier property, etc., the stretch-formed plastic containers obtained by using a polyester as represented polyethylene terephthalate (PET) and the like have now been widely used for a variety of applications.
In recent years, further, it has been strongly urged to reutilize the resources, and the foamed and stretched containers have been known as the stretch-formed containers to favorably meet such a demand. Namely, the foamed and stretched containers are capable of expressing light-blocking capability due to foaming without being blended with a coloring agent, and is very adapted to being recycled as compared to the containers which are imparted with the light-blocking capability by being blended with the coloring agent.
As means for foaming plastic formed bodies, the chemical foaming has long been known using a chemical foaming agent such as sodium carbonate. At present, however, attention has been given to the foaming based on the microcellular technology according to which an inert gas is dissolved in a plastic material and is grown into bubbles without using the chemical foaming agent. This foaming technology is also called physical foaming and offers such advantages as forming the bubbles (foamed cells) in sizes considerably smaller than those of the chemical foaming and enabling the bubbles to be homogeneously distributed.
The foamed and stretched plastic containers that utilize the foaming based on the microcellular technology have been proposed, for example, by the present applicant (patent documents 1 to 4).
As the methods of producing stretched plastic containers, there have been known the cold-parison method and the hot-parison method.
The cold-parison method is a method of producing containers by forming container preforms by injection-molding a plastic material, once cooling the preforms, and stretch-forming the preforms by transferring them to the step of stretch-forming such as blow-molding. In this method, the step of forming the preforms by the injection-molding is completely separate and independent from the step of forming the preforms into the containers by the stretch-forming, offering a great advantage from the standpoint of high-speed production and mass production, such as making it possible to set optimum conditions in each of the forming steps and to operate the forming steps at their maximum rates. This method, further, offers such advantages that the preforms can be stored, that the site where the containers which are the final products are to be produced can be determined depending upon the requirements of the users without being restricted to the site where the preforms are produced. Therefore, the PET bottles for containing, especially, beverages have almost all been produced by the cold-parison method.
On the other hand, the hot-parison method is a method of producing containers by stretch-forming the container preforms formed by injection-molding the plastic material without cooling the preforms, and transferring them to the step of stretch-forming while maintaining them at a stretchable temperature. In this method, the preform that is formed is readily stretch-formed without interruption making it possible to utilize, in the step of stretch-forming, the heat possessed by the preform readily after it is formed offering a great advantage from the standpoint that the heat energy is effectively utilized and that the facility cost is not expensive. Therefore, this method is advantageous for the production of thick containers to which the cold-parison method could not be applied since it was difficult to heat the preforms. In this method, however, the stretch-forming is conducted right after having formed the preforms and, therefore, the stretch-forming conditions are dependent upon the conditions for forming the preforms (e.g., dependent upon the forming rate). The mass productivity and the rate of production are inferior to those of the cold-parison method and, therefore, the hot-parison method is applied to producing products in many kinds but in small lots (e.g., containers for containing seasonings and detergents).
It is very difficult to apply the hot-parison method to the production of foamed and stretched plastic containers by utilizing the microcellular technology.
Namely, with the cold-parison method, the preform that is formed is once cooled and is, thereafter, stretch-formed. It is, therefore, allowed to provide the step of foaming by heating between the step of forming the preform and the step of stretch-forming, and to control the degree of foaming by adjusting the heating condition. With the hot-parison method, on the other hand, the stretch-forming is conducted following the formation of the preform without interruption. Therefore, it is not allowed to provide any independent step of foaming between the step of forming the preform and the step of stretch-forming, and it is very difficult to control the foaming.
In producing foamed containers, for instance, it is required to suppress the foaming in the mouth portion of the containers to which a cap is to be fixed by fitting or screw-engagement. This is because a change in the size due to foaming, an increase in the surface roughness and a decrease in the strength, bring about a decrease in the sealing by the cap and, further, make it difficult to attain the engagement between the cap and the mouth portion of the containers.
In fact, a patent document 5 proposes a foamed and stretched plastic container obtained by the hot-parison method without, however, teaching anything about suppressing the foaming in the mouth portion of the containers. Therefore, the foamed container is very poorly practicable.